The present invention relates to wireless communication systems, and more particularly, to a method and apparatus for efficiently controlling power levels in a mobile radio.
In a typical cellular radio system, a geographical area is divided into cell areas served by base stations which are connected to a radio network. Each user (mobile subscriber) in the cellular radio system is provided with a portable, pocket, hand-held, or car mounted mobile station which communicates voice and/or data with the mobile network. Each base station includes a plurality of channel units including a transmitter, a receiver, and a controller and may be equipped an omni-directional antenna for transmitting equally in all directions or with directional antennas, each directional antenna serving a particular sector cell. Each mobile station also includes a transmitter, a receiver, a controller, and a user interface and is identified by a specific mobile station identifier. Each mobile subscriber is identified by another identifier, e.g., an international mobile subscription number (IMSI).
The present invention is described in the non-limiting, example context of a universal mobile telecommunications (UMTS) 10 shown in FIG. 1. A representative, connection-oriented, external core network, shown as a cloud 12 may be for example the Public Switched Telephone Network (PSTN) and/or the Integrated Services Digital Network (ISDN). A representative, connectionless-oriented external core network shown as a cloud 14, may be for example the Internet. Both core networks are coupled to corresponding service nodes 16. The PSTN/IDSN connection-oriented network 12 is connected to a connection-oriented service node shown as a Mobile Switching Center (MSC) node 18 that provides circuit-switched services. In the existing GSM model, the MSC 18 is connected over an interface A to a Base Station Subsystem (BSS) 22 which in turn is connected to radio base station 23 over interface Axe2x80x2. The Internet connectionless-oriented network 14 is connected to a General Packet Radio Service (GPRS) node 20 tailored to provide packet-switched type services sometimes referred to as the serving GPRS service node (SGSN). Each of the core network service nodes 18 and 20 connects to a UMTS Terrestrial Radio Access Network (UTRAN) 24 over a radio access network (RAN) interface. UTRAN 24 includes one or more radio network controllers 26. Each RNC 26 is connected to a plurality of base stations (BS) 28 and to any other RNCs in the UTRAN 24.
Preferably, radio access is based upon wideband, Code Division Multiple Access (WCDMA) with individual radio channels allocated using CDMA spreading codes. Of course, other access methods may be employed. WCDMA provides wide bandwidth for multimedia services and other high transmission rate demands as well as robust features like diversity handoff and RAKE receivers to ensure high quality.
The mobile stations 30 use transmission codes so base station 28 can identify transmissions from that particular MS 30. In the current WCDMA standard, codes are supposed to be allocated as follows for the dedicated channels:
a) the uplink and downlink transmission is using channelization codes, and on top of that a scrambling code;
b) the channelization code determines e.g., the spreading factor, and the spreading factor determines the maximum bitrate;
c) mobiles in the same cell using the same frequency and the same spreading factor use different channelizations codes for the downlink channels but the same scrambling code; and
d) mobiles in other cells use the same channelization codes but different scrambling codes.
The scrambling codes secure the integrity, between downlink transmissions using the same channelization code but in different cells. The scrambling code used in uplink secure the integrity between uplink transmissions from different mobile stations in the same or in other cell.
Thus, the MS gets its own scrambling code while the BS transmission to a specific mobile on a dedicated channel will use a common scrambling code but a unique channelization code. The MS have the ability to combine a downlink transmission using a different scrambling codes and different channelization codes (one limitation today is that the spreading factor of the channelization codes must be the same from all cells).
The radio network controller 26 and base station 28 shown in FIG. 2 are radio network nodes that each include a corresponding data processing and control unit 32 and 33 for performing numerous radio and data processing operations required to conduct communications between the RNC 26 and the mobile stations 30. Part of the equipment controlled by the base station data processing and control unit 33 includes plural radio transceivers 34 connected to one or more antennas 35. The mobile station 30 shown in FIG. 3 also includes a data processing and control unit 36 for controlling the various operations required by the mobile station. The mobile""s data processing and control unit 36 provides control signals as well as data to a radio transceiver 37 connected to an antenna 38.
The present invention may be employed in the context of the example mobile communications system 10 shown in FIG. 1 in which the radio network controllers 26 and base stations 28 form a radio access network between a core network node (like the MSC 16) and the mobile stations 30.
It is important for the mobile stations 30 to maintain appropriate power levels when communicating with the base station 28 in order to prevent one mobile station from overwhelming communications from other mobile stations in the transmission area. Because the power level for mobile stations is a critical parameter for maintaining good communication quality within a particular cell, is valuable for power control to be performed as often as possible. Ideally, each mobile station is continually monitored to ensure that its power levels are high enough to provide good transmission quality yet no higher than necessary to provide that transmission quality and no higher than will create unreasonable interference with other mobile station communications. The invention also applies to maintenance of proper power level in the base station transmission.
In prior systems, very fast power control of mobile station communications was typically performed by the network in FIG. 1 using signal-to-interference (Eb/Io) measurements. The signal-to-interference measurements were typically performed over a couple of pilot symbols contained in each slot. The measured Eb/Io for the uplink slot is compared to a target and Transmission Power Control (xe2x80x9cTPCxe2x80x9d) bits in the next downlink slot are set to order a one step increase or decrease of the mobile station power.
Although signal-to-interference measurements can be made very rapidly, and therefore, are useful in fast power control, varying and different propagation conditions cause the Eb/Io parameter to be less than accurate in determining whether a mobile station should be commanded to increase or decrease its transmit power. There are several possible reasons for this inaccuracy:
If the Eb/Io is changing too fast, e.g., due to a fast moving mobile, the power control delay will be too large and the power adjustment wvill come too late to be able to counteract the changed Eb/Io.
Also, the measured Eb/Io estimate will not be valid for the whole slot since the true Eb/Io changes during non-measured time periods during a slot.
The radio propagation conditions, e.g., rapid variation in the number of radio paths used, will affect the Eb/Io estimate in a negative manner.
In addition to the inaccuracy of the Eb/Io based power control, the Eb/Io estimate""s inability to reflect the true, end user perceived quality also creates a problem. The end-user perceived quality is more accurately estimated by using estimated frame error rate.
To increase accuracy, it has been proposed that frame error rate (FER) be used as a more accurate power control parameter. Frame error rates identify the accuracy of frame transmissions and are good indicators of power control. Unfortunately, FER parameters are slowly calculated so power control is not quickly achieved using FER as a measurement.
To illustrate this difference, FIG. 4 shows a WCDMA frame protocol. Each mobile station MS communicates with a base station BS using the WCDMA frame protocol shown. The uplink physical control channel is used to carry control information including known pilot bits to support channel estimation for coherent detection, transmit power-control (TPC) commands, feedback information (FBI), and an optional transport-format combination indicator (TFCI). The transport-format combination indicator informs the receiver about the instantaneous parameters of the different transport channels multiplexed on the uplink channel, and corresponds to the data transmitted in the same frame.
FIG. 4 shows the frame structure of the uplink dedicated physical channels. Each frame of length 10 ms is split into 16 slots, each of length Tslot=0.625 ms, corresponding to one power-control period. A super-frame corresponds to 72 consecutive frames, i.e., the super-frame length is 720 ms.
Controlling the power level between the mobile station MS and the base station BS is a continual process. If it is done bit-by-bit (or symbol-by-symbol), in accordance with the Eb/Io formula, the mobile station will receive very fast power control correction, but such corrections may not be particularly accurate. For the example embodiment of FIG. 4, each frame of 10 millisecond duration includes 16 slots, so an Eb/Io measurement each slot yields an Eb/Io value correction every 0.625 milliseconds to adjust power by a predetermined increment (for example, 1 dB). On the other hand, if a frame error rate is used, power control is better able to maintain a desired end user perceived quality during semi-static conditions, but occurs too slowly since just one frame error determination requires at least the 10 ms needed to receive the frame. For a fast moving mobile station, slow power correction can cause a significant problem where the mobile continually ends up in high power/low power conditions which remain uncorrected for significant durations. In addition, the proximity of the estimated Eb/Io to the true Eb/Io changes with changing radio conditions, as does the mapping between Eb/Io and end user qualityxe2x80x94both of which are effected by first moving mobiles.
Consequently, Eb/Io provides fast power control, but is not particularly accurate at determining appropriate power conditions for subsequent transmissions. Frame error rates provide more accurate power control, but occur too slowly. The present invention utilizes a function determined by Eb/Io (fast but less accurate), bit error rate BER (slower but more accurate), and FER (still slower but still more accurate) as a method of determining power control. The present invention applies equally to single links and plural links (soft handovers). The present invention also applies to both/either power control of base station transmitters and/or mobile station transmitters. Thus, although the present description is provided by reference to power control of the uplink (ISS transmitter), the invention covers and applies to power control of the downlink (base station transmitter) as well.